Retroviruses exhibit a wealth of evolutionary phenomena, including the ability of replicating populations to undergo rapid genetic change in response to varying selective pressure;the ability to vary in the use of host cell receptors;and the ability to become integrated in the genome of their host species and passed down through the generations as endogenous proviruses. In the prior project period, we have engaged all these aspects of retrovirus evolution: We have studied the evolution of env genes by analyzing unusual mutants that extend the host range of ALV beyond chicken, to quail, dog, and even human cells. We have analyzed the evolutionary pathway as well as the novel biochemical mechanism involved. We have extensively analyzed the coevolution of retroviruses and their hosts, both in humans and in mice. We have probed the role of an important antiviral host factor (AP0BEC3G) in the evolution of both endogenous MLV and exogenous HIV. We have developed sophisticated mathematical models for the evolution of replicating virus populations, describing the effects of mutation, selection, drift, linkage, and recombination on the accumulation (or loss) of deleterious mutations, and begun development of an in vitro model system to apply these models to the real world. Future work will continue these studies, with the following aims. 1. How do retroviral envelope genes evolve from receptor independence to use new receptors and to alter other important properties? 2. What are the functional and pathogenic properties of human endogenous retroviruses, particularly HERV-K? Are any of these elements still active and capable of replication and integration at new sites in the human genome? 3. How do important forces of mutation, selection, recombination, and drift combine to direct retrovirus evolution?